1. Field of the Invention
The present invention is directed to a method and system for determining an endpoint of a process in a semiconductor system, and more particularly to a method and system for using laser interferometry for endpoint detection.
2. Discussion of the Background
The fabrication of integrated circuits (IC) in the semiconductor industry typically employs plasma to create and assist surface chemistry within a plasma reactor necessary to remove material from and deposit material to a substrate. In general, plasma is formed within the plasma reactor under vacuum conditions by heating electrons to energies sufficient to sustain ionizing collisions with a supplied process gas. Moreover, the heated electrons can have energy sufficient to sustain dissociative collisions and, therefore, a specific set of gases under predetermined conditions (e.g., chamber pressure, gas flow rate, etc.) are chosen to produce a population of charged species and chemically reactive species suitable to the particular process being performed within the chamber (e.g., etching processes where materials are removed from the substrate or deposition processes where materials are added to the substrate).
Although the formation of a population of charged species (ions, etc.) and chemically reactive species is necessary for performing the function of the plasma processing system (i.e. material etch, material deposition, etc.) at the substrate surface, other component surfaces on the interior of the processing chamber are exposed to the physically and chemically active plasma and, in time, can erode. The erosion of exposed components in the plasma processing system can lead to a gradual degradation of the plasma processing performance and ultimately to complete failure of the system.
Semiconductor processing facilities require constant monitoring. Processing conditions change over time with the slightest changes in critical process parameters creating undesirable results. Small changes can easily occur in the composition or pressure of an etch gas, process chamber, or wafer temperature. Semiconductor processing is an example of a manufacturing process that requires close tolerances of the width of features produced on the wafer. It is important that the width of etched features during processing be controlled within certain critical dimension limits. In many cases, changes of process data reflecting deterioration of processing characteristics cannot be detected by simply referring to the process data displayed. Oftentimes statistical process control (SPC) is necessary to detect these slight changes.
Plasma etching is a fundamental technique used in integrated circuit fabrication. It is very important to determine when the endpoint of the etching process has been reached. If processing is stopped prior to the endpoint some of the etched layer remains. If over-etching occurs, lower layers of the integrated circuit wafer could be damaged. It is important to know when the nominal endpoint has been reached to achieve accurate and repeatable etching.